This invention relates generally to methods and apparatus for computed tomography imaging, and more particularly to methods and apparatus for computed tomography imaging with physiological gating to reduce motion artifacts and patient radiation exposure.
In at least one known computed tomography (CT) imaging system configuration, an x-ray source projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system and generally referred to as the "imaging plane". The x-ray beam passes through the object being imaged, such as a patient. The beam, after being attenuated by the object, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is dependent upon the attenuation of the x-ray beam by the object. Each detector element of the array produces a separate electrical signal that is a measurement of the beam attenuation at the detector location. The attenuation measurements from all the detectors are acquired separately to produce a transmission profile.
In known third generation CT systems, the x-ray source and the detector array are rotated with a gantry within the imaging plane and around the object to be imaged so that the angle at which the x-ray beam intersects the object constantly changes. A group of x-ray attenuation measurements, i.e., projection data, from the detector array at one gantry angle is referred to as a "view". A "scan" of the object comprises a set of views made at different gantry angles, or view angles, during one revolution of the x-ray source and detector. In an axial scan, the projection data is processed to construct an image that corresponds to a two dimensional slice taken through the object. One method for reconstructing an image from a set of projection data is referred to in the art as the filtered back projection technique. This process converts the attenuation measurements from a scan into integers called "CT numbers" or "Hounsfield units", which are used to control the brightness of a corresponding pixel on a cathode ray tube display.
In one known system, to reduce artifacts to an acceptable level, an entire scan is performed in a fraction of cardiac cycle with an electron beam CT (EBCT) imaging device. In an EBCT imaging device, x-ray emissions generated by a scanning electron beam are used for imaging. Physical rotation of a gantry is eliminated, and a scan of the electron beam can be completed in as little as 50 milliseconds to essentially completely freeze cardiac motion for coronary imaging. However, the known EBCT imaging device is considerably more expensive than conventional CT imaging devices and is not available in many hospitals. Moreover, the known EBCT imaging device images only a single slice of a volume at a time. Thus, repeated radiation doses are required to provide comprehensive three-dimensional coverage of a volume, squandering at least a substantial portion of any potential reduction of patient radiation dose.
It would therefore be desirable to provide methods and apparatus for computed tomography imaging that reduce motion artifacts utilizing less expensive radiation systems. It would also be desirable to provide such methods and apparatus that reduce patient radiation exposure during imaging.